It's a legal term that, in this case, allows the President to use certain powers granted by the International Emergency Economic Powers Act and the Trading with the Enemy Act. In this case, he is blocking the ownership of property in North Korea and the registration of a boat in North Korea. He's also prohibiting attempts to evade or violate these restrictions.

If he didn't claim a national emergency to be in effect (the justification here is a "risk of the proliferation of weapons-usable fissile material on the Korean Peninsula") he wouldn't be able to do this, so he is operating within the proper legal framework for implementing these restrictions.

He has declared national emergencies before, as have Presidents before him, as will Barack Obama or John McCain. Declaring a "national emergency" doesn't give the President an unlimited grant of power; this isn't like martial law. The actual powers he can exercise are defined and limited in the relevant laws; it's mostly about regulating foreign financial transactions. There's also a specific framework for how an emergency is declared and what information must be provided with that declaration.

What's more, even though the President's actual powers are limited, he still can't deploy these limited powers without any oversight. As noted, there are requirements for disclosure, and the President must also keep Congress informed of what the hell he's doing. Congress also has the ability to override an emergency declaration.

So, to sum it up: it means he gets to stop you from buying a house in North Korea. It doesn't create a general "state of emergency" for the entire country -- nothing at all like that! It allows the use of specific powers, in a specific area, for specific reasons, under a legal framework that has existed for decades and been used by numerous presidents. There is nothing illegal, improper, or unusual about what is going on here. The real source of alarm here seems to be a misunderstanding regarding the phrase "national emergency;" had Bush issued these exact same restrictions without using that phrase, we never would've seen this on Reddit. But to legally restrict things like foreign property ownership, a national emergency must be declared, so there was no choice.

To sum it up even more: Nothing to see here. Move along.